1. Field of the Invention
The present invention relates to a plasma processing apparatus, and more specifically, it relates to a plasma processing apparatus that does not allow charged particles of plasma generated in a processing chamber to enter a gas supply unit.
2. Description of the Related Art
Plaza processing apparatuses in the known art include those that execute plasma processing such as etching on the work surface of a workpiece, e.g., a semiconductor wafer (hereafter simply referred to as a “wafer”) placed within a processing chamber by, for instance, supplying a processing gas from a gas supply unit into the processing chamber and generating plasma with the processing gas.
The gas supply unit in such a plasma processing apparatus is constituted as a shower head having numerous gas supply holes through which the processing gas is supplied into the processing chamber. The plasma processing apparatus may be, for instance, a plane parallel plasma processing apparatus having a lower electrode disposed within the processing chamber, on which the workpiece is placed. The gas supply unit is constituted of a shower head also functioning as an upper electrode which is disposed at the ceiling of the processing chamber so as to face opposite the lower electrode.
The gas supply unit includes an electrode plate that constitutes the lower surface thereof, in which numerous gas supply holes are formed and an electrode support body supporting the electrode plate. Inside the electrode support body, a buffer chamber is formed as a space located above the electrode plate and communicating with a gas supply pipe, and the buffer chamber also communicates with the gas supply holes at the electrode plate. The gas flowing in through the gas supply pipe is first supplied into the buffer chamber and is then guided from the buffer chamber into the processing chamber via the gas supply holes at the electrode plate.
However, charged particles such as electrons and ions in the plasma generated with the processing gases inside the processing chamber may enter the buffer chamber through the gas supply holes at the gas supply unit in the plasma processing apparatus. If charged particles in the plasma enter the gas supply unit (shower head), a glow discharge occurs in the buffer chamber at the gas supply unit, giving rise to problems such as reaction products becoming adhered to the inner surfaces of the gas supply unit and the inner surfaces of the gas supply unit becoming corroded.
These problems are addressed in, for instance, Japanese Patent Laid-open Publication No. 9-275093, which discloses a structure achieved by mounting a screw having a hole decentered from the central axis at each gas outlet hole of the gas supply means so that there is no clear passage from one opening end of the gas supply hole through the other opening end to prevent entry of electrons and ions in the plasma into the gas supply means. This technology was developed in order to minimize the entry of charged particles through the mean free path based upon the concept that the charged particles in the plasma are allowed to enter the gas supply means since the thickness of the electrode plate (the height of the gas supply holes) is approximately equal to the length of the mean free path of the charged particles in the plasma.
However, the charged particles in the plasma enter the gas supply means not only through the mean free path but also because of other factors. For instance, the potential (the ground potential) at the electrode support body constituting the upper wall of the buffer chamber in the gas supply unit may become lower than the potential (ground potential) at the electrode plate constituting the lower wall of the buffer chamber. In such an event, the charged particles in the plasma are allowed to readily enter the buffer chamber from the gas supply holes at the electrode plate toward the electrode support body. In addition, while the gas supply unit normally maintains a field free state inside, the equipotential line will become skewed at an end of a gas supply hole and shifts into the gas supply hole if the gas supply hole is clear, thereby allowing a concentration of energy of the electrons and the like and allowing the electrons and the like to readily enter the gas supply hole.
For this reason, charged particles in the plasma cannot be fully prevented from entering the gas supply means simply by mounting a screw having a hole decentered from the central axis at each gas outlet hole of the gas supply means as disclosed in Japanese Patent Laid-open Publication No. 9-275093. For instance, since high-frequency power causes charged particles such as electrons to vibrate along a direction perpendicular to the equipotential line, the oscillating direction of the charged particles becomes tilted if the equipotential line becomes skewed and shifts into the end portion of the gas supply hole. In such a case, the entry of the charged particles cannot be fully prevented simply by mounting a screw having a hole decentered from the central axis.
Furthermore, entry of the charged particles in the plasma is most likely to occur when various conditions such as a specific gas supply hole diameter, a specific gas type and a specific plasma density coincide. This leads to a concept that if the gas passage at the gas supply hole can be altered in correspondence to predetermined conditions, the entry of the charged particles in the plasma into the gas supply unit can be prevented more effectively.
Accordingly, an object of the present invention, which has been completed by addressing the problems discussed above, is to provide a plasma processing apparatus capable of fully preventing charged particles in the plaza generated inside the processing chamber from entering the gas supply unit.